1. Field of the Invention
The present invention relates generally to ultrasonic cleaning. In particular, the present invention relates to ultrasonic cleaning of the interior surfaces of elongated tubes. The United States Government has fights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Pipes or tubes for circulation of process fluids or containment of materials are abundant in industrial facilities. To avoid contamination of chemical compounds used in a process, all pipes, containers, and so forth to be used in the process are thoroughly cleaned before use. In particular, the surfaces of process piping must be cleaned to remove deposits of grease and particulate materials remaining from machining operations.
A number of techniques are available for cleaning elongated tubes. Cleaning is often accomplished simply by submerging the tubes in a cleaning solution for a predetermined time, or by pressure flushing with chlorinated fluorocarbon (CFC) solvents such as methylene chloride toluene. CFC solvents are effective but environmentally undesirable, so their use is being reduced or eliminated in many areas. Aqueous, non-CFC, solvents are biodegradable, thus are considered to be environmentally preferable. However, aqueous solvents are much less effective cleaning agents than CFC solvents when used with these conventional techniques.
High pressure, low-frequency shock waves are used to unplug blocked pipes (Simon, U.S. Pat. No. 4,974,617; Coon et al., U.S. Pat. No. 4,551,041), and clean corrosion products and sedimentation from the interior walls of heat exchanger tubes (Scharton et al., U.S. Pat. No. 4,645,542). Such techniques are, however, not suitable for cleaning the interior surfaces of elongated tubes for the purpose of degreasing, due to the high pressures (up to 5,000 psi) of the shock waves and extended time periods required (1-24 hours).
The use of ultrasonics to enhance the cleaning effectiveness of solvents is well known. Ultrasonic techniques are particularly valuable when aqueous solvents are used, since aqueous solvents are intrinsically less effective than CFC solvents. The object to be cleaned is placed in a bath containing a mixture of water or some other solvent. Ultrasonic waves agitate the mixture, inducing cavitation or fluid breakdown at sites where the localized pressure is low enough that the fluid can no longer support the sound wave. At typical ultrasonic frequencies, cavitation occurs at sound pressures of approximately 0.36 watt/cm.sup.2 in water, compared to approximately 1.20.times.10.sup.3 watts/cm.sup.2 in air at atmospheric pressure. The mechanical disruption and agitation of the fluid at the cavitation sites significantly enhances its effectiveness as a cleaner and degreaser.
John, Jr. et al. describe an ultrasonic cleaning apparatus for fuel rod tubes in U.S. Pat. No. 4,966,177. These perforated tubes are conveyed through a cleaning bath filled with detergent-containing water. Ultrasonic transducers induce cavitation within the water, thereby cleaning the tubes. This technique is also effective for cleaning the exterior surface of a solid wall tube. However, metals reflect a significant fraction of incident sound energy, so the wall of the tube forms a barrier to efficient energy transfer to the interior. Thus, cavitation cannot readily be induced within the interior of the solid wall of the tube when it is immersed in an ultrasonic bath, so there remains a need for a method for the effective cleaning of the interior surface of a tube.